Effect of microstructure and phosphorus content on the corrosion behaviour of electroless nickel-phosphorus coatings

The effects of microstructure and phosphorus (P) content on the corrosion characteristics of electroless Ni-P coatings on mild steel in a 3.5 wt% NaCl solution were investigated using a potentiodynamic technique. The results indicate that the anodic and cathodic behaviour of the coatings varied with changes in the structure, morphology and phosphorus contents of the coatings. Different alloy compositions and heat treatment conditions influenced both the microstructural characteristics and crystallization behaviour of the deposit. The addition of thiourea to the electroless plating solution increased the intrinsic stress due to the stabilizing effect on the bath. It is postulated that the coating should enjoy enhanced corrosion resistance for applications where mild steel is in contact with a saline environment.